Process for casting fused refractory oxides having high melting points

ABSTRACT

The present invention provides a process and apparatus for the production of refractory oxide materials having high fusion points. By high fusion points is meant melting points greater than 2400° C. 
     The process of the present invention comprises the steps of: 
     (a) insulatingly supporting a vertically elongated bed of particulate refractory oxide material having a high fusion point at its bottom and sides with additional amounts of the particulate refractory oxide which will remain unmelted through the process, 
     (b) exposing the upper surface of the bed to a plurality of electrodes to form at least one arc adjacent to the upper surface of the bed, 
     (c) melting a portion of the bed by the flow of electric current from one electrode to another, forming a zone of liquid refractory oxide material, or bath, surrounded by a layer of congealed material, or skull, 
     (d) removing a portion of particulate material along the side of the bed to expose the congealed layer at a point adjacent the bottom portion of the zone of liquid material, 
     (e) piercing the congealed layer and tapping the zone of liquid material, and 
     (f) tilting the bed to pour molten refractory oxide material from said liquid zone to a point outside of the bed. 
     The present invention also provides a furnace for the production of fused refractory oxide materials having high fusion points. The apparatus comprises: 
     (a) a furnace shell having a bottom and at least one side wall adapted to receive and contain a charge of the refractory material, 
     (b) the side wall of the shell having a vertical notch, or slit, therein extending from the top of the wall downward to a distance at least 2/3 of the total height of the shell, 
     (c) the notch adapted to be sealed by a removable closure means and, when unsealed, to receive a tapping means, such as a ram or a jet tapper, 
     (d) a pouring spout positioned adjacent the notch, extending outward from the shell, 
     (e) means to heat the interior portion of the shell, and 
     (f) means to tilt the shell to an angle from about 5° to about 90° from the initial, at rest, position.

This is a division of co-pending application Ser. No. 45,485 filed June4, 1979, now U.S. Pat. No. 4,304,954.

BACKGROUND OF THE INVENTION

The present invention relates to a process and apparatus for the castingof fused refractory oxide materials having high fusion points, such asmagnesia. Such refractory oxide materials have melting points greaterthan about 2400° C. Many refractory oxide materials, such as alumina,have relatively low melting points and are commonly commerciallyproduced on a continuous or semi-continuous basis by pouring or tappingthe molten oxide from a furnace. However, such methods have not beensuccessfully employed with refractory oxides having melting points overabout 2400° C. because the fluidity of such material decreases at a veryrapid rate with a small decrease in temperature, causing the material tosolidify in the furnace or to a great extent during pouring before anysignificant amount of product can be poured or drawn from a tap.

The present method and apparatus are particularly adapted to theproduction of fused magnesia (MgO). The melting point of magnesia isabout 2800° C. Fused magnesia is typically commercially produced bybatch processes using electric arc furnaces of the open arc type fromfeed material of calcined magnesia or magnesite having a high MgOcontent.

Conventionally fused magnesia is produced by placing a metal container,suitably a steel shell, partially filled with a bed of feed material,such as calcined magnesia, under a set of graphite or carbon electrodes.A bridge of carbon or graphite, suitably in the form of resistor bars,is positioned in the bed to form an electrical connection, or bridge,between the electrodes when they are lowered. Current passing throughthe bridge causes local heating and subsequently the formation of aninitial molten oxide pool. When the pool is of a size sufficient toconduct electric current by itself, the electrodes are moved to aposition sufficiently close to the surface of the pool so that electriccurrent passes through the arc to the molten pool. The container isfilled gradually feeding additional feed material in the area of themolten bath. As the molten area increases in size, a surrounding layerof feed material congeals to form a container for the fused magnesia.This layer is generally termed the "skull". Between the skull and thesides of the container, a second layer of partly solidified material isformed, which, in turn, is surrounded by a layer of substantiallyunaffected feed material. The skull layer and the surrounding layers ofunfused feed material serve to insulate the shell from the hightemperatures of the bath. To further cool the shell, water is sprayed onthe outside portions during the fusion operation and, if desired, duringthe cooling stage.

Typically, fused magnesia is formed, in situ, by allowing the contentsof the furnace to cool sufficiently to solidify the bath. The fusedmagnesia product, usually in the form of a large mass, generallyreferred to as a "pig", is removed from the furnace after cooling bychipping with pnuematic hammers or, more frequently, by dumping thefurnace contents on a sorting floor and separating the product from thecongealed material and the unfused material. The congealed material andthe unfused material are recovered and utilized as feed material insubsequent fusions. The pig is broken into smaller pieces as a feedmaterial for a crusher or a subsequent milling operation. The product,granules or grains of fused magnesia, is useful in the production ofshaped refractory materials by being bonded together, with or withoutadditional refractory materials, shaped and fired. The particulateproduct is also useful as an electrical insulator material in tubularheating elements, such as those used in electric stoves, water heaters,cooking pots and frying pans.

The working of the pig, i.e., separating and breaking the pig to obtainpieces suitable for crushing, is time-consuming and requires asubstantial amount of arduous labor.

The present invention provides a method and apparatus for thesemi-continuous production of a cast, fused magnesia product. The term"semi-continuous" means that more than one run, or pour, of fusedmagnesia may be produced from a furnace without removal of the skullfrom the furnace. The cast product may be produced in a thin layerhaving a relatively smooth surface, particularly adapted to being easilyfractured into pieces suited to crushing and requires no separation offused from unfused material.

BRIEF DESCRIPTION OF THE INVENTION

The present invention provides a process and apparatus for theproduction of fused refractory oxide material having high melting orfusion points. By high fusion points is meant melting points greaterthan 2400° C.

The process of the present invention comprises the steps of:

(a) insulatingly supporting a vertically elongated bed of particulaterefractory oxide material having a high fusion point at its bottom andsides with additional amounts of the particulate refractory oxide whichwill remain unmelted through the process,

(b) exposing the upper surface of the bed to a plurality of electrodespositioned over said bed to form at least one arc adjacent to the uppersurface of the bed,

(c) melting a portion of the bed by the flow of electric current fromone electrode to another, forming a zone of liquid refractory oxidematerial, or bath, surrounded by an open-topped layer of congealedmaterial, or skull,

(d) removing a portion of particulate material along the side of the bedto expose the congealed layer at a point adjacent the bottom portion ofthe zone of liquid material,

(e) piercing the congealed layer and tapping the zone of liquidmaterial, and

(f) tilting the bed to pour molten refractory oxide material from saidliquid zone to a point outside of the bed.

The present invention also provides a furnace for the production offused refractory oxide materials having high fusion points. Theapparatus comprises:

(a) a furnace shell having a bottom and at least one side wall adaptedto receive and contain a charge of the refractory material,

(b) the side wall of the shell having a vertical notch, or slit, thereinextending from the top of the wall downward to a distance at least 2/3of the total height of the shell,

(c) the notch adapted to be sealed by a removable closure means and,when unsealed, to receive a tapping means, such as a ram or a jettapper,

(d) a pouring spout positioned adjacent the notch, extending outwardfrom the shell,

(e) means to heat the interior portion of the shell, and

(f) means to tilt the shell to an angle from about 5° to about 90° fromthe initial vertical, or at rest, position.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will be further illustrated and described indetail by reference to the accompanying drawings and the followingexamples which are to be considered illustrative of the invention andnot limiting.

FIG. 1 is a side view, partly in section, of a furnace of the presentinvention.

FIG. 2 is a frontal view, partly in section, of the furnace of FIG. 1.

FIG. 3 is a frontal view, partly in section, of the furnace of FIG. 1being tapped.

FIG. 4 is a frontal view, partly in section, showing the furnace of FIG.1 in a pouring position.

FIG. 5 is a frontal view, partly in section, showing the furnace of FIG.1 being tapped after several previous tappings.

Looking now at FIGS. 1 and 2, the furnace is comprised of a metal shell11, suitably of steel, having an elongated notch, or slit, 13 and acorresponding pouring spout 15, positioned adjacent notch 13 andextending outward from the side wall of shell 11. The outside and bottomof shell 11 is suitably cooled with a water spray, not shown. Thefurnace contains a charge 17 of particulate refractory oxide materialhaving a high melting or fusion point, e.g., magnesia, which has beenheated by electrodes 19 positioned to heat the interior of the furnaceand to form a bath, or zone, 21 of molten refractory oxide material. Askull layer 23 of soidified, or congealed, refractory oxide material isformed around the zone of liquid refractory oxide material. A secondarylayer of partially sintered and partially fused refractory oxidematerial 25 is formed around the skull layer. The partially sinteredlayer, in turn, is surrounded by layer 27 of unmelted refractory oxidematerial. The vertical elongated bed of refractory oxide material issupported at its bottom and sides by metal shell 11 and, in turn,insulates shell 11 by layers 27, 25 and 23 of unmelted refractory oxidematerial. Elongated notch 13 is suitably sealed by a removable,water-cooled damming plate 29.

FIG. 3 illustrates the furnace of FIG. 1 being tapped. Damming plate 29has been removed from notch 13, and a portion of the unfused refractoryoxide layers 17 and 25 adjacent notch 13 has been removed, and trench 31formed in particulate charge layer 17 and partially sintered layer 25. Atap 33 is suitably made by a rod 35 or by a jet tapper using anexplosive charge, penetrating skull layer 23 preferably adjacent thelower portion of notch 13.

FIG. 4 shows the tapped furnace of FIG. 3 tilted to cast a fusedrefractory oxide product. The furnace is tilted, by means not shown, toan angle between about 5° and about 90° and, more preferably, betweenabout 15° and about 60° from the original, vertical position, allowingmolten refractory oxide material 37 to flow through tap 33 in skull 23,through trench 31 in layers 25 and 17 to a point outside furnace shell11. Preferably, tilting is carried out as speedily as possible toproduce a rapid flow of the molten refractory oxide material beforesolidification.

FIG. 5 shows the furnace of the previous drawings being tapped afterseveral previous tappings. As in FIG. 3, damming plate 29 and a portionof the unfused refractory oxide layers 25 and 17 have been removed andtrench 31 formed therein. As shown, the residues 39, 41 and 43 fromprevious tappings 45, 47 and 49 require that each new tapping be placedprogressively higher within notch 13. Suitably, the tappings are carriedout at angles progressively upward, that is, greater from horizontal, toinsure tapping the molten zone within the furnace and to avoid anyprevious taps and any bottom build up of solidified refractory oxidematerial, such as 39, 41 and 43, which may have been formed within thezone of liquid refractory oxide material from previous pours.

EXAMPLE 1

A 24 inch diameter steel shell furnace similar to that shown in FIG. 1was equipped with two 4-inch diameter graphite electrodes adapted tooperate at a voltage of 82 volts. The center to center distance of theelectrodes was 9 inches.

The furnace was filled 3/4 full with calcined magnesia and the meltstarted by means of a graphite starting bar with the electrodes pulledclose together. After getting a melt and an average energy input of atleast 100 KW, the electrodes were pulled apart.

Thirty minutes after the start, a full energy input of 150 KW wasattained, and the furnace operated at this level for one hour. At thispoint, the furnace was slightly inclined at an angle of about 15°, and aportion of the loose charge and partly congealed material removed toexpose the skull layer. A "V" shaped trench was made in the loose chargeand partly congealed layer to form a path for the molten magnesia. A11/2 inch diameter, eight foot long steel rod was used as a tapping ramto pierce the skull layer. The ram was removed and the furnace tilted toan angle of 60° to remove molten magnesia from the furnace.

EXAMPLE II

In this example a 900 KW average input single phase arc furnace havingtwo 14 inch diameter electrodes waw used. The center to center distancebetween the electrodes was 22 to 24 inches . The distance between theelectrodes and the side wall of the furnace was about 10 inches. Thefurnace notch was sealed with magnesia brick, and the furnace wascharged with calcined magnesia.

After an initial heating of about 21/2 hours, the magnesia brick wasremoved, and a portion of the adjacent loose charge and partly congealedmaterial removed to expose the skull layer. A trench was prepared in theremaining loose material and partly congealed material to receive themolten magnesia.

A steel rod 2 inches in diameter about 10 feed in length was used as atapping ram. The ram was suspended by a wire rope to permit swinging theram to acquire the necessary kinetic energy to pierce the skull layer.Piercing the skull required from 2 to 5 hits with the ram.

After piercing, the ram was quickly removed, and the furnace tilted toan angle of about 60° to pour the melt from the furnace and preventsolidification of magnesia in the tap hole and pouring spout.

After each pour, additional feed material was added to the furnace, andthe tapping procedure was repeated at progressively higher points in theskull about every 21/2 hours, yielding pours of from about 500 to about800 pounds of fused magnesia. After the procedure was repeated 24 times,the bottom crust build-up became too thick to continue.

While there have been described various embodiment of the invention, themethod and apparatus described is not intended to be understood aslimiting the scope of the invention as it is realized that changestherewithin are possible, and it is intended that each element recitedin any of the following claims is to be understood as referring to allequivalent elements for accomplishing the same results in substantiallythe same or equivalent manner, it being intended to cover the inventionin whatever form its principle may be utilized.

What is claimed is:
 1. A process for the production of fused refractoryoxide materials having fusion points greater than 2400° C. whichcomprises the steps of:(a) insulatingly supporting a verticallyelongated bed of said refractory oxide material in particulate form atits bottom and sides with additional amounts of said particulaterefractory oxide material which will remain unmelted through theprocess, (b) exposing the upper surface of said bed to a plurality ofelectrodes positioned over said bed to form at least one arc adjacent tothe upper surface of said bed, (c) melting a portion of said bed by aflow of electric current from one of said electrodes to another, forminga zone of liquid refractory oxide material surrounded by an open-toppedlayer of congealed refractory oxide material, (d) removing a portion ofparticulate material along the side of said bed to expose said congealedlayer at a point adjacent to the bottom portion of said zone of liquidmaterial, (e) piercing said congealed layer and tapping the zone ofliquid material, and (f) tilting said bed to pour molten refractoryoxide material from said liquid zone to a point outside of said bed. 2.The process of claim 1 wherein the refractory oxide material ismagnesia.
 3. The process of claim 1 wherein the elongated bed ispositioned within a metal shell.
 4. The process of claim 3 wherein themetal shell is cooled by water spray during steps (c) and (d).
 5. Theprocess of claim 1 wherein step (d). The bed is tilted to an anglebetween about 5° and about 90°.